FIELD OF THE INVENTION
The invention relates to a contact configuration for vacuum switches, including two mutually opposite switch parts or contact pieces having a circular base surface with a predetermined radius and a central circular recess, each switch part or contact piece having three to six slots proceeding from a center and extending outward, so that a spiral contact is formed, and the contours of the slots having a variable radius.
Vacuum switches have become increasingly used during recent times. Besides the medium voltage range, applications are also being proposed in the high voltage and low voltage range. In the case of the latter range, the contact configurations in the vacuum switching tubes are mainly constructed in such a way that a radial magnetic field to influence an arc is created.
A contracted metal vapor arc which is created in the case of short-circuit interruptions in vacuum switching tubes with radial magnetic field contacts should move away as fast as possible from its place of creation in the central region of the contacts. Local thermal overloads of the contact surfaces are avoided in that way. A large thermal stress due to a fast continuous running of the arc in the marginal zones of the switch parts or contact pieces should be prevented in the further temporal progression of the interruption as well. Moreover, the outwardly directed plasma stream from the arc should be as small as possible in order to keep the screen loading small.
In the case of spiral contacts, such an arc behavior is achieved, for example, by the application of slots in the switch parts. Through the use of the structure of those slots, it is possible to compel such a direction of the current flow in the contacts that the magnetic field caused thereby or the Lorentz force associated therewith influences the arc in its movement behavior and plasma escape behavior in an appropriate manner.
In the case of known spiral contacts which are used in commercial vacuum switching tubes, difficulties occur either in the course of the free running of the arc or in the course of the subsequent circulation in the contact marginal zones, as well as in the course of the transition between the two movement phases. Moreover, the commutation of the arc across the slots may be made more difficult if the magnitude of the driving magnetic force is indeed large but the azimuthal component moving the arc is too small and the radial component driving the plasma stream onto the screen is too large. In the case of those examples, either the slots extend in a rectilinear manner or their contour is distinguished by a constant radius of curvature.
U.S. Pat. No. 4,999,463 contains a mathematical description of a slot contour which possesses a variable radius, the foot of which does not coincide with the center of the switch part. As a result thereof, the slot can extend as far as the recess of the contacts, and in that way persistence of the arc in the course of free running can be avoided. In that case, however, the radial and azimuthal components of the Lorentz force acting on the arc at the outer end of the spiral wing seem to be so large and so small respectively that on one hand its commutation is made more difficult, and on the other hand the thermal screen loading becomes excessively large. Moreover, the free running of the arc is made more difficult by the small angle of intersection which exist between the slot contour and the periphery of the recess.